1. Field of the Invention
The present invention relates to a ceramic composite material which has a high mechanical strength and an excellent creep resistance in a wide temperature range from room temperature to high temperature and is suitable to be used as a structural material at a high temperature.
2. Description of the Related Art
SiC or Si.sub.3 N.sub.4 has been investigated to develop ceramic materials to be used at high temperatures but is not sufficient in high temperature properties. As an alternative material thereof, SiC/SiC composite materials produced by chemical vapor impregnation, provided by Societe Europeene de Propulsion, have attracted attention, at the present are considered to be the best high temperature structural materials and have been investigated and developed. The temperature range at which they can be used is reported to be 1400.degree. C. or lower.
Mr. T. Parthasarathy et al. in Journal of the American Ceramics Society Vol. 76, No. 1, pp29-32 (1993) disclosed a composite of alumina and yttrium aluminum garnet (sometime hereinafter referred to as "YAG") of eutectic Al.sub.2 O.sub.3 -Y.sub.3 Al.sub.5 O.sub.12. Mr. Parthasarathy et al. also disclose a method of producing the composite by unidirectionally melting and solidifying a mixed powder of Al.sub.2 O.sub.3 and Y.sub.2 O.sub.3 in a crucible.
It is comprehensible from the description on page 29, right column, lines 9 to 10 and FIGS. 1 and 2 of the literature that the composite is polycrystalline and includes grain boundaries. This is clearly supported from the description "The failure was usually along the colony boundaries with cracks running along the Al.sub.2 O.sub.3 -YAG interface boundaries". These colony boundaries are shown as portions where the microstructure is larger than in the other portions in FIG. 2 of the literature.
This composite material has stresses equivalent to those of sapphire at 1530.degree. C. and 1650.degree. C. when the strain rate was made constant.
Moreover, the present inventors confirmed in experiments that the composite disclosed by Mr. Parthasarathy et al. includes pores or voids in the microstructure and the mechanical strength of the composite rapidly declares at high temperature.
Therefore, the object of the present invention is to provide a ceramic composite material comprising sapphire, single crystalline .alpha.-Al.sub.2 O.sub.3, and YAG, which has excellent mechanical strength and creep behavior from room temperature to high temperature and significantly improved properties particularly at high temperature.